Partial ileal bypass has been proven to be the most effective way of reducing serum cholesterol in humans as well as in experimental animals. In our laboratory we were able to demonstrate this serum cholesterol-lowering effect in cholesterol-fed animals. Serum cholesterol decreased from a preoperative value of 160 mg/100 ml to 60 mg/100 ml six weeks after operation during continued maintenance on a cholesterol-rich diet. Our preliminary results also indicate that the serum Lecithin: Cholesterol Acyltransferase (LCAT) is elevated 6-fold after partial bypass in cholesterol-fed animals. Because this increase of LCAT is coincident with the cholesterol-lowering effect, the influence of partial ileal bypass on cholesterol metabolism and its relation to LCAT will be studied in this project. Lipoproteins, which are substrates for LCAT, will be isolated and characterized in normal and in cholesterol-fed animals, and the influence of partial ileal bypass on these lipoproteins will be studied. Lipoprotein Lipase, another enzyme which regulates the levels of serum lipoprotein also will be studied before and after the operation. If the mechanism of the cholesterol-lowering effect of this intestinal bypass is found to be due to other biochemical alterations which the bypass introduces into the system, methods other than surgical procedure to lower the serum cholesterol may be developed.